Natural selection generally acts on phenotypes, but is ineffective without genetic variation between individuals in a population. Genetic correlations between characters are dure to udiquitous pleiotropic (manifold) effects of genes, and linkage disequilibrium (non random associations of alleles affecting different traits). These are particularly important in determining the response to selection, or random genetic drift in small populations, because genetically correlated characters cannot evolve independently. Currently popular models used to analyze selection and evolution of correlatec characters assume that the regression of offspring on parent phenotypes is linear, and that the characters have a multivariate normal distribution in the population. The effects of nonlinear (nonadditive) inheritance caused by genetic interaction, i.e. dominance and epistasis, on phenotypic evolution will be investigated. The joint influence of genetic interation, linkage disequilibrium and nonrandom mating on the inheritance and evolution of quantitative characters will also be investigated theoretically. Experiments with Dorsophila melanogaster will be preformed to assess the extent of non linearity in offspring-parent regressions and response to selection for morphological characters and female fecundity. Spontaneous mutation affecting morphological characters and fecundity in parthenogenetic (asecual) strains of Drosophila mercatorum will also be studied experimentally. These studies will help to clarify fundamental questions concerning the evolution of correlated characters in natural populations, and will also have applications in pest or disease control programs and in artificial selection experiments.